Depreciation on pro rata basis

Andy has a business that sells stationeries and toys. On 1st June 2013, he bought an equipment with
a list price of $33,000. He also paid $2,000 to deliver the equipment to his shop and $1,000 to install
and test it. A further $2,000 was paid for the first-year maintenance. He expects to use the
equipment for 10 years after which its residual value is estimated at $1,000. He decided to
depreciate the equipment at 20% per annum, using the reducing balance basis.

On 1st May 2016, Andy decided to change the depreciation method for equipment to straight-line
method. He also decided to revise the estimated residual value to $2,336 and to decrease the
remaining useful life of the equipment by 2 years.
Andy charges depreciation on pro-rata basis.

Later, at the end of its useful life, Andy traded in the equipment with a new one that costs $25,000.
He paid $21,500 by cheque to the equipment supplier

**Question: What is the carrying value of the equipment as at 30th April 2019?

I do get the first part where you depreciate using reducing balance basis:

36000×0.2×11/12= 6600
29400×0.2 =5880
23520×0.2 =4704
Accumulated dep=17184
Carrying value as at 1st may 20×6=18816

For straight line basis:
(18816-23360)/??
**This where im stuck at, should i divide with 60 or 61 months?

Thank you in advance.